At 11.58 hours on 10 Jun 1942, U-68 fired one torpedo from a distance of about 2000 meters at the unescorted Port Montreal (Master John Godfrey Lewis) about 178 miles north of Cristobal, Panama. The wake was spotted on the ship and she turned to evade but because of that the torpedo hit the stern and caused the ship to sink fast. Merten noted in the war diary that the shot was out of desperation and the hit was very lucky.

At 19.30 hours on 8 June, the Port Montreal had picked up 43 survivors from Tela which had been sunk by U-504 (Poske) earlier that day. The master, 42 crew members, two gunners and the 43 survivors abandoned ship in four lifeboats but two men from Tela later died in the boats and were buried at sea. On 16 June, the survivors were picked up by the Colombian schooner Hiloa and landed at Cristobal the next day.